Journal of Hydrology 500 (2013) 144–156 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol Interaction of esker groundwater with headwater lakes and streams ⇑ Pertti Ala-aho , Pekka M. Rossi, Bjørn Kløve University of Oulu, Department of Process and Environmental Engineering, Water Resources and Environmental Engineering Laboratory, P.O. Box 4300, 90014 Oulu, Finland article info summary Article history: Groundwater–surface water interactions were studied in a Finnish esker aquifer system where some Received 7 February 2013 lakes suffer from periodic water level decline and others from eutrophication. Natural tracers (Ca2+, Received in revised form 11 June 2013 3À SiO2 and PO ) were used on the aquifer scale and seepage meter measurements and water level record- Accepted 3 July 2013 4 ings on the single lake scale to better understand the complex interactions between groundwater, lakes Available online 17 July 2013 and streams in the area. The natural tracers showed that lakes located in lower landscape positions and This manuscript was handled by Laurent Charlet, Editor-in-Chief, with the assistance connected with streams were richer in tracer concentrations and that their water chemistry resembled of P.J. Depetris, Associate Editor that of the groundwater. On the other hand, closed basin lakes located at higher altitudes were nutri- ent-poor seepage lakes with some groundwater through flow. The data suggests that the subsurface acts Keywords: as a phosphate source for the surface water bodies and the nutrient rich groundwater inseepage dictates Groundwater–surface water interaction the trophic status of the lakes in the area. The seepage meter measurements verified a strong interaction Seepage meter between groundwater and surface water and revealed spatial distribution of lake seepage and a temporal Natural tracers co-variation in lake seepage rates. A statistical analysis of seepage meter and water level observation time Groundwater flow systems series were useful to show groundwater flow regimes controlling lake seepage. Using the natural tracer analysis and seepage meter measurements, we developed a novel conceptual model for the study site where the differences in lake trophic state and water table behavior were explained by assigning lakes to local and regional groundwater flow regimes. The obtained results provide new knowledge on using natural tracers in complex glacial aquifer systems and can be used in integrated groundwater–surface water management. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction (Winter et al., 1998): (1) Drainage lakes that receive groundwater inflow throughout their bed and are usually located in topograph- Eskers are abundant in the Fenno-Scandinavian shield and are ically low-lying areas, in the regional groundwater discharge area. common in other regions covered by the last glaciation. Esker aqui- (2) Recharge lakes that recharge groundwater throughout their bed fers are often connected to rivers, lakes and wetlands and such and often occur topographically high in the groundwater recharge groundwater-dependent systems are of high ecological value area. (3) Seepage lakes located between the extreme cases above (Kløve et al., 2011). Eskers consist geologically of permeable sandy and that can receive groundwater inflow in some lake sections soils and gravels, so surface runoff is usually minor. Instead, water and lose water to groundwater in others. The concept of local ponds in landscape depressions as a result of subsurface flow, and regional groundwater systems, first introduced by Toth forming closed basin lakes and wetlands. As such lakes are usually (1963), has provided theoretical background for several aspects embedded in the aquifer and their water level and water chemistry of lake-groundwater interaction. Following Winter (1976), many are highly dependent on the groundwater system (Winter et al., studies suggest that lake position with respect to local and regional 1998). In addition to the physical characteristics of the lakes, flow systems may determine a number of factors in their hydrol- groundwater exchange affects ecosystems by providing nutrients, ogy and chemistry. inorganic ions and a stable water temperature (Hayashi and Rosen- Despite growing interest in groundwater–surface water interac- berry, 2002). tions, few field studies have sought to explain the hydrological In glacial terrain, such as eskers, lakes interacting with ground- conditions or water quality of lakes by their position with respect water can be roughly categorized into three types with a typical to local or regional groundwater flow systems. Most field methods position in the landscape and in the groundwater flow system for studying groundwater–surface water interactions aim to quan- tify the water flux at the interface, disregarding the fact that groundwater inseepage to lakes may differ in solute concentrations ⇑ Corresponding author. Tel.: +358 294484498. and inflow stability depending on the groundwater flow system. E-mail addresses: pertti.ala-aho@oulu.fi (P. Ala-aho), pekka.rossi@oulu.fi However, Winter and Carr (1980) used numerical groundwater (P.M. Rossi), bjorn.klove@oulu.fi (B. Kløve). 0022-1694/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2013.07.014 P. Ala-aho et al. / Journal of Hydrology 500 (2013) 144–156 145 modeling to relate lakes with high concentrations of total dissolved systems surrounding lakes with data from well-established field solids to regional or deep intermediate groundwater flow systems measurements. Firstly, we used natural tracers to pinpoint lakes and lakes with lower concentrations to local or small intermediate and streams to different flow systems according to their tracer con- flow systems. Aquifer scale flow systems were also studied by centration, water body type and landscape location. Secondly, we Webster et al. (1996) who concluded that the landscape position demonstrate methods for separating groundwater flow systems of a lake can control its ionic composition and may influence its surrounding a single lake by statistical analysis, using data from chemical response to droughts or other climatic changes. A study seepage meters and standard hydrogeological observations. With by Kratz et al. (1997), conducted in the same lake system, also the methods presented, we developed a conceptual model for the attributed the biological composition of lakes to their position in groundwater flow system that could explain anomalous behavior the landscape. of lakes co-occurring in similar geological and land-use settings: In comparison with studies examining lake systems on the a periodically declining water level in some lakes and a eutrophic aquifer scale, studies distinguishing between local and regional state in others. origins of groundwater inflow to lakes are more abundant on the scale of a single lake. Researchers report cases where changes in re- gional groundwater systems affect water and nutrient fluxes in 2. Materials and methods individual lakes (see e.g. Brock et al., 1982; Anderson and Cheng, 1993; Winter, 1999; Hagerthey and Kerfoot, 2005). In addition, 2.1. Study site many studies have recognized groundwater as an important source of nutrients and inorganic ions in lakes (Shaw et al., 1990; Krab- The Rokua esker aquifer is part of a long esker ridge stretching benhoft and Webster, 1995; Devito et al., 2000; Hayashi and inland from the North Ostrobothnian coast (Aartolahti, 1973) and Rosenberry, 2002; Sebestyen and Schneider, 2004; Hagerthey and was formed during the last deglaciation some 9000–12,000 years Kerfoot, 2005). ago (Tikkanen, 2002). Eskers are long, narrow formations of sand Kenoyer and Bowser (1992) provided a theoretical background and gravel associated with the retreat of the ice cover (Svendsen to the chemical evolution of groundwater along different ground- et al., 2004). Hydrologically, the Rokua esker aquifer is a very com- water flow systems and showed that longer subsurface flow paths plex, unconfined aquifer system with interdependent groundwater, lead to increased alkalinity and increased concentrations of some lakes and streams in the aquifer recharge area and groundwater-fed major cations. The starting hypothesis for our investigations at springs and streams in the groundwater discharge area (Fig. 1). aquifer scale was that the chemical constitution of lake water in According to groundwater head observations, the aquifer water ta- terms of natural tracers (silica, calcium and phosphate) can be ex- ble has two regional groundwater mounds. Groundwater flows plained by the position of the lake with respect to groundwater from the areas of high water table in recharge zone in a radial fash- flow systems as the contact time with water and soil in the regio- ion towards the low-laying discharge zones (Fig. 1). nal groundwater system can be assumed to be longer than in small In geological terms, the aquifer material is fine and medium local flow systems (Kenoyer and Bowser, 1992). In addition, at the sand derived from sedimentary rock of the Muhos formation scale of a single lake, we used natural tracer analyzes to look for (Pajunen, 1995) and consists mainly of quartz. Thickness of the similarities between the quality of lake water and that of ground- sand deposits varies from 30 m to more than 100 m above the bed- water adjacent to the lake. rock. In addition, some loam lenses and gravel deposits, typical of Of the selected analytics, phosphate is